reading subjects into variables

Hi.

I want to have a procmail recipe that will save certain mail to folders 
where the folder name (always a number) is specified in the subject.

I tried:

:0

* Subject: [0-9]*
$HOME/folders/`formail -x Subject:`

and that doesn't work (file gets saved to folders directory but under 
a strange filename - a different filename every time).

I realize that I must have an error in the formail command.  What is 
it, though?

Any help will be greatly appreciated.

Thanks,

Sara